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Friction is a very important factor in all phases of
knitting. In case of yarn friction applies to resistance developed by yarn
sliding over another yarn or over metallic or ceramic bodies. There are two
types of friction 1) static 2) kinetic. Resistance offered by the yarn in
contact with guides bars or eyelets. When the machine is started is an example
of static friction while movement of yarn through the guides or needles when
the machine is running is kinetic friction. Static friction is generally
greater than kinetic friction. Frictional resistance or coefficient of friction
depends on many factors such as:

i.Surface
smoothness

ii.Yarn color

iii.Yarn twist

iv.Package hardness

v.Moisture content
in the yarn

vi.Yarn lubricants used

vii.Yarn tension etc

Surface Smoothness: It is generally assumed that
smoother the surface lesser is the coefficient of friction. However it has been
found in the case of synthetic yarns that the smooth filaments of these
materials stick to smooth surface and produce a large area of contact and
therefore large frictional drag. But the mat surface presents less area of
contact reducing the coefficient of friction. Of course the mat surface should
be only microscopically visible.

Yarn Color: All factors remaining same when color is
changed it results in change of dimensions. Darker shades give more friction
increased tension and hence lower stitch length. Dyed yarn has usually a higher
coefficient of friction via 0.30.

Yarn Twist: Increased twist in yarn reduces friction.
Low twist yarns spread out and have increased area of contact.

Moisture Content: Moisture improves pliability of yarn
and makes it more plastic so that more regular and uniform loops are formed.
Moisture enables knitting with ease. Moisture content in the yarn should be 10
to 40 per cent. Small changes in moisture do not change friction as such. Wide
differences in moisture however do cause differences in dimensions due to
changes in frictions.

Yarn Lubricants: As mentioned earlier yarns are
lubricated to reduce friction before supplying it to the machine. A suitable
selected lubricant for a given fiber and guide surface may cut the coefficients
of friction to half its value. What is equally important is that yarns with
different initial coefficient of friction can be brought to a comfortable low
friction level after lubrication thus avoiding variations in quality. The
quality of lubricant and the amount of lubricant to be put on yarn is also very
important. Between 1/2 and 2 per cent lubricant gives good results. The
conventional method of applying lubricant is to place a paraffin wax dist
between the metallic discs used as tensioner as shown in figure. This method is
however inefficient as grooving of the wax causes variations in wax application
disc. A more efficient method is the application of wax emulsion as shown in
figure.

Yarn Tension: When the yarn is knitting on the machine
kinetic friction is developed. The measure of yarn friction is calculated by
measuring the input and output tensions and using the Amontons equation To
=Ti.eµƟ where to is the output tension Ti is the input tension
e=2.718 µ is coefficient of friction and Ɵ is the angle of warp in radians
shown in figure. Output tension thus depends on the input tension coefficient
of friction and the angle of contact. It has been estimated that during the
formation of any one loop the yarn must pass over as many as a dozen guide
surfaces and due to the friction involved the tension in the yarn increases.
The increased tension causes the loop length to change and thus fabric quality
is changed. It has been found that if the coefficient of yarn friction is less
than 0.2 faults in knitting usually associated with friction are mostly eliminated.
Spun yarns like wool when not waxed will have values of µ=0.4.Such high values
cause excessive yarn failure.

If for
example coefficient of friction =0.50 and the angle =90º =Π/2 then,

To=
Ti.eµƟ 0.5 ×1.57

= Ti × 2.718

= 2Ti (approx)

Thus
the output tension becomes double the input tension under such conditions. If
the yarn is warping partly round a number of guides then the tension is built
up progressively. A 3-g input tension may rise up to 300-g output tension up to
knitting point. It’s therefore necessary to test the yarn friction regularly
before knitting. A number of instruments are now available to test the yarn
friction. The use of positive fee reduces to some extent the detrimental effect
of high yarn friction knittability. As a rough guide both for staple and
filament yarns the optimum yarn input tension is found to be about 10% of the tex number of the yarn.
However it is also recommended that the input tension does not drop below 1.5g
otherwise difficulties arise in setting such low tensions. The coefficient of
kinetic friction increases with the yarn speed. Thus a 15 denier nylon yarn
passing through chromium plated guides has coefficient of friction = 0.35 at
about 100 meters per minute.